In general, electric-discharge machining is a process of melting a portion of a work piece using an electric-discharge phenomenon and removing the molten portion from a nonmolten portion in order to machine a desired shape of the work piece. Specifically, in electric-discharge machining, the work piece and a tool-shaped electrode suitable for the desired shape as a cutting tool are arranged spaced apart in a dielectric machining fluid. A voltage of approximately 100V is applied there between, while a distance there between is gradually closed. At that time, a spark discharge is generated at a surface of the electrode or work piece. A material of the work piece is molten or evaporated at a discharging point where the spark discharge is generated, and the work piece adjacent to the discharging point is removed by pressure of the dielectric fluid. Fine craters are continuously formed by the discharge to perform cutting.
A turbine used in an engine for industrial applications or aircraft is a mechanical component used to obtain a rotational force from a combustion gas or pressurized air by compressing air. Such a turbine may be manufactured through the above described electric discharge. One example of a method for manufacturing a turbine blade using the electric-discharge machining is disclosed in Korean Patent Laid-Open No. 2004-9436 (“the '9436 application”). A turbine provided according to the '9436 application is shown in FIG. 1. The turbine includes a disc 10 of a round plate and a plurality of blades 20 spaced apart from at regular intervals. According to the '9436 application, a plurality of reference elongated grooves 15 are formed along a flange of a disk material 1, as shown in FIG. 2. An upper right surface 11 and an upper left surface 12 are formed on the basis of the elongated grooves 15, as shown in FIG. 3, to form an upper portion relative to a symmetrical line S. After turning over and fixing the disk material, a second right surface 13 and a second left surface 14 are formed on the basis of the elongated grooves 15 to form a second portion relative to the symmetrical line S.
According to the '9436 application, in the case where there is no overlay between the blades 20, as shown in FIG. 4A, in other words, in the case of a>b, reference elongated grooves may be formed at a blade gap 10, and a tool electrode may be vertically moved to perform cutting working, as indicated by symbol P in FIG. 4A. However, when there is an overlay between the blades 20, as shown in FIG. 4B, in other words, when a≦b, since the gap between the blades 20 is not sufficient, it is not possible to form reference elongated grooves. In addition, a tool electrode is not vertically moved to perform cutting working.
A turbine with an integral shroud, in which a shroud integrally formed with outsides of the blades, cannot be manufactured through a conventional method of forming the reference elongated grooves 15 by moving the tool electrode from a cross section of the disk material 1 to the inside, as shown in FIG. 2.
Meanwhile, since the blades of the turbine are formed in a C-shape, a complete set of blades cannot be obtained through one electric-discharge machining. Accordingly, the blades are formed by electric-discharge machining the upper and second portions divided on the basis of the symmetrical line (S in FIG. 3). In this case, there is a problem in that a mismatch occurs between the upper portion and the second portion, as shown in FIG. 5. An electric-discharged machined portion at a boundary between the upper portion and the second portion is not smoothly joined and a protrusion or a stepped portion at a machined surface between the upper and second portions is formed. This obstructs a flow of fluid, such as combustion gas, passing thorough the blades, thereby causing an efficiency of the turbine to deteriorate.
In addition, one blade is formed by one tool electrode, and after the disc material is rotated in a circumferential direction, a next blade is formed by the one tool electrode. The turbine is electric-discharge machined by repeating the above process as many times as the number of the blades. Therefore, excessive machining time is required to form a plurality of blades. Also, there is another problem in that since it is difficult to keep a setting of each blade consistent; precision of the machining is seconded.